1. Field of the Invention
The invention relates to a method for determining an image of a first portion of a body placed in a steady magnetic field by means of magnetic resonance (MR), which method comprises the following steps;
excitation of nuclear spins in the first portion, PA0 measurement of a data set of MR signals of the first portion of the body, and PA0 reconstruction of the image from the measured data set, the method also comprising the generation of an RF pulse rotation of a magnetization in a second portion of the body. The invention also relates to an MR apparatus for carrying out such a method, comprising; PA0 means for generating temporary magnetic gradient fields, PA0 means for generating RF pulses, PA0 means for receiving MR signals, PA0 processing means for the reconstruction of an image of the MR signals received, PA0 control means for the generation of control signals for the means for generating temporary magnetic gradient fields and for the means for generating RF pulses, PA0 the control means also being arranged to generate an MR imaging sequence comprising an RF excitation pulse and an RF pulse for rotation of a magnetization within the second portion of the body. PA0 a first magnetic field gradient pulse is generated with the first selective RF pulse according to a first gradient function of time, PA0 a second magnetic field gradient pulse is generated with the second selective RF pulse according to a second gradient function of time being a time, reversed version of the first gradient function, PA0 a sign of the second gradient function opposes that of the first gradient function, PA0 the first selective RF pulse is applied according to a first amplitude modulation function of time and a first frequency modulation function of time, PA0 the second RF pulse is applied according to a second amplitude modulation function and a second frequency modulation function, PA0 the second amplitude modulation function is a time reversed version of the first amplitude function, PA0 the second frequency modulation function is a time reversed version of the first frequency modulation function, and PA0 a sign of the second frequency modulation function opposes to that of the first frequency modulation function. In this way, for example, a highly selective inversion pulse can be obtained by the concatenation of the first selective RF pulse and the second RF pulse, because a flip angle of the z-magnetization is exactly doubled at all positions within the selected slice. RF-pulses having a large flip angle can thus be derived from RF-pulses having a small angle without loss of profile quality. For the concatenation of the first and the second selective RF pulse a slice quality of for example, 3.5 percent can be obtained. Whereas the slice quality of a slice selected by, for example, a conventional 180 degree inversion RF pulse amounts to 7 percent. An example of such a conventional inversion RF pulse comprises an asymmetric amplitude envelope. PA0 the RF pulse for 180 degree rotation of the magnetization is generated after an excitation RF pulse and PA0 in that a further magnetic field gradient pulse is applied in the direction normal to the selected slice. The concatenation of the first selective RF pulse and the second RF pulse can be applied as a refocusing pulse in, for example a known Echo Planar Imaging (EPI) imaging sequence or in a known spin echo imaging sequence or gradient and spin echo sequence (GRASE). These imaging sequences are known per se from international patent application WO 93/01509. In the known spin echo sequence or EPI sequence the concatenation of the first and second RF pulses is generated after the excitation of the nuclear spins for a 180 degree rotation of the magnetization in the selected slice in order to rephase the spins in the selected first portion so as to generate an MR signal. The area of the magnetic field gradient pulse depends on the first gradient RF pulse and on the modulation function of the first and second RF pulses.
2. Description of Related Art
Such a method is known from U.S. Pat. No. 5,402,785. The known method is used for the magnetic resonance imaging of a part of a body of an animal or human. The MR images obtained may be used as a tool for the diagnosis of a decease of the animal or human by determining perfusion in, for example a part of the brain. Perfusion images show motion of blood in vessels and capillaries in tissue of the human or animal under examination. The known method comprises the determination of perfusion by labelling atoms in a fluid in the second portion located upstream with respect to a flow of liquid towards the first portion, the generating of a steady state in the substance by continuing to label atoms until the effect caused by the labelled atoms perfusing in the substance reaches a steady state, and the generating of image information so as to determine perfusion in the first portion of the body. The labelling involves continuous inversion of spins associated with the atoms in the second portion. In this way the second portion in the form of a slice is selected for which a Larmor frequency of the spins in the slice equals the frequency of the RF pulse. A drawback of the known inversion pulse is that the obtained slice profile deviates from a nominal selection profile, for example a rectangle.